stdio: Provide functions to add/remove devices using stdio_dev

stdio: Provide functions to add/remove devices using stdio_dev

The current functions for adding and removing devices require a device name.
This is not convenient for driver model, which wants to store a pointer to
the relevant device. Add new functions which provide this feature and adjust
the old ones to call these.

Signed-off-by: Simon Glass <sjg@chromium.org>